This invention relates to a method for preparing radium-free standard solutions of radon-222-in-water (the radon-222 being referred to as Rn hereafter) of known concentration for use in calibrating liquid scintillation counters. A radium-free radon-in-water standard of the type contemplated herein is defined as a known volume of deionized water with a known activity of radon gas without the presence of any radium activity.
Rn is radioactive gas which is formed during the radioactive decay of radium-226 (referred to as Ra hereafter). Ra is found in trace concentrations in many rock formations. It has a long half-life of about 1620 years. Rn, on the other hand, has a half-life of only 3.823 days.
Rn gas found in the air in energy-efficient dwellings has raised concern because of its carcinogenic effects. It is believed that most lung cancer among non-smokers is caused by exposure to excessive concentrations of Rn in indoor air. Health statisticians reportedly attribute 5,000 to 30,000 lung cancer deaths per year to Rn gas exposure. These figures, which are less than fatalities caused by cigarette smoking, but greater than those caused by asbestos, have generated a strong demand from the scientific community to produce accurate reproducible, quantitative and qualitative measuring techniques for Rn.
Rn is relatively soluble in water (51 cc of Rn per 100 cc of water in cold water) whereas Ra sulfate is virtually water-insoluble (less than 2 micrograms of Ra sulfate per 100 cc of water at 25 degrees C.). Thus, due to the proximity of ground water to geological strata that bear a precursor to Rn, i.e., Ra, it is therefore believed that well water is one of the mediums of transport for Rn into indoor air.
Current methods for detecting Rn in water using liquid scintillation measurements are calibrated mostly using Ra standards. Ra, however, is not likely to be present in water in the same abundance of Rn due to its relatively low solubility in water as compared to Rn. Ra and Rn standards do not produce identical spectrums in liquid scintillation (FIG. 1) and it is the spectrum produced that are used as the basis to determine the nuclide concentration. Therefore, liquid scintillation systems calibrated with Ra standards will not detect the activity as accurately as liquid scintillation systems calibrated with radium-free Rn standards.
Two known methods are principally used for determining Rn concentration in water: gamma spectroscopy and liquid scintillation counting. Gamma spectroscopy can detect and quantify individual radionuclides with a high degree of resolution. However, it involves long count times and the sampling procedure is cumbersome. Gamma spectroscopy, therefore, is a relatively slow and expensive method which is impractical when many samples of ground water must be analyzed for Rn content in a short time. Gamma spectroscopy, however, detects only gamma emitters. Because Rn is a pure alpha emitter and the gamma spectrometer can only detect gamma emitters, the activity of Rn is determined indirectly through the activity of its gamma emitting progeny, Bismuth-214 and Lead-214.
With that procedure, the coaxial Ge(Li) (a Germanium drifted Lithium) detector of the gamma spectrometer is calibrated using a National Bureau of Standards (NBS) traceable mixed gamma solution for a fixed Mason jar geometry, which is mentioned hereinafter.
Due to Rn's relatively short half-life of 3.823 days, the activity of the sample must be determined within one week of obtaining the sample. Samples analyzed later than this period will have decayed to the point that the original activity cannot be accurately detected. Therefore, the use of gamma spectroscopy analysis with a large number of samples is impractical.
On the other hand, as stated above, because Rn is a pure alpha emitter, it can be detected directly by the liquid scintillation method; therefore liquid scintillation counting is a relatively fast and inexpensive method for determining Rn concentration in water. The liquid scintillation system can count numerous samples in a short period of time and involves small sampling containers. However, liquid scintillation counters must be calibrated with Ra-free Rn-in-water standard solutions of known concentration. The method of generating said standards is the principal object of the present invention.
A prior art method of preparing Ra-free-Rn-in-water standard solutions is described by the National Bureau of Standards (NBS) in an article authored by Hutchinson, J. M. R., et. al., Nuclear Instruments and Methods in Physics Research A247 (1986) (385-389) in which a known amount of Ra is encapsulated in a plastic tube and inserted in a glass chamber filled with water. However, a mathematical relationship is used in their prior art system to calculate the Rn concentration in the solutions based on the diffusion coefficient between Rn and the plastic tubing in relation to time. Their apparatus must be carefully and thoroughly purged of Rn so that at time zero there is no Rn in the water in the accumaltor or in the plastic tubing holding the Ra. Also, a system constant must be established for each system, referred to as the source emanation power which depends on the relative masses of water and plastic in the system, and also on the solubility and diffusability of Rn in the plastic. This prior art method is exceedingly tedious, cumbersome and vulnerable to error.